Due to the development of wireless MTC technology, the number of cellular MTC subscribers has increased significantly during the past several years. This machine-type communication brings in some new features and requirements for air-interface optimization. The 3rd generation partnership project (3GPP) has both structure analysis (SA) and radio access network (RAN) working groups working on these new features for optimizations. Some common features have been figured out in 3GPP TR 23.888 by SA2, such as small-data transmission, lower-power consumption, time-controlled traffic, infrequent traffic and so on. RAN 2 has studied RAN improvements for MTC and RAN1 is studying low-cost MTC terminals based on LTE.
FIG. 1A shows the contention based random access procedure. FIG. 1B shows a diagram of a MAC random access response message. In 3GPP TS 36.300, four steps of the contention based random access procedures are described, which is performed when a UE initially accesses the network. UE selects a random access preamble based on eNB's configuration through system information and transmits this preamble on a time and frequency resource. Random Access Response (Msg2) conveys at least the RA-preamble identifier, Timing Advance command, initial UL grant and assignment of Temporary C-RNTI, as shown in FIG. 1A. The UE sends the scheduled transmission (Msg3) through the initial UL grant (as shown in FIG. 1B) conveyed in Msg2. For initial access, Msg3 conveys the RRC Connection Request generated by the RRC layer and at least the NAS UE identifier. Msg4 conveys contention resolution. HARQ feedback is transmitted only by the UE which detects its own UE identification, as provided in message 3, echoed in the Contention Resolution message.
Compared with the data package size, the signaling overhead (PDCCH/MAC CE/RRC message) is relatively large. Finding methods of more efficiently transmitting the small data with less overhead is an important issue.